The naxx cost is for the PvE content shit. If they added a set of 100cards, you wouldn't be losing shit, they'd just drop from packs and arena runs like usual and you'd craft them with dusts like usual. They could make the new cards special so that you can't buy them with current ressources and force people to regrind, but I'd assume they'd just have a shittier conversion rate instead or something like that. Also it isn't really like a mmo where you add more powerful cards, you just add more different cards. Having a new deck type like say paladin secret deck wouldn't instantly invalidate the current decks like handlock or miracle rogue, so you wouldn't be at a disadvantage besides in mirror if there's a really strong card for your particular deck, but you'd just need to focus on getting that. And obviously regardless of what they add it would have no effect for arena players.